Integrated and combined phase shifter and isolation switch

ABSTRACT

A phase shifter unit cell or a connected set of such cells that can be well isolated from external circuitry and which do not introduce insertion loss into an RF signal path, exhibit good return loss, and further provides additional advantages when combined with bracketing attenuator circuits. More particularly, embodiments integrate a high-isolation function within a phase shifter circuit by breaking the complimentary nature of the control signals to a phase shifter cell to provide greater control of switch states internal to the phase shifter cell and thus enable a distinct high-isolation state, and by including a switchable shunt termination resistor for use in the high-isolation state. Some embodiments are serially coupled to attenuator circuits to enable synergistic interaction that reduces overall die size and/or increases isolation. One such embodiment positions a high-isolation phase shifter cell in accordance with the present invention between bracketing programmable attenuators.

BACKGROUND

(1) Technical Field

This invention relates to electronic radio frequency (RF) circuits, andmore particularly to RF phase shifter circuits.

(2) Background

Electronic phase shifter circuits are used to change the transmissionphase angle of a signal, and are commonly used to phase shift RFsignals. RF phase shifter circuits may be used for applications such asin-phase discriminators, beam forming networks, power dividers,linearization of power amplifiers, and phased array antennas, to name afew.

For many applications, it may be useful to serially-connect multiplephase shifter unit cells of the same or different phase shift values.Such phase shifter circuits may be digitally controlled and thus providea discrete set of phase states that are selected by a binary controlword, directly or after decoding. For example, such phase shiftercircuits may be binary-coded, thermometer coded, or a hybrid combinationof the two types. Some phase shifter circuits may also include adigitally controlled RF signal attenuator circuit that provides adiscrete set of attenuation states that are selected by a binary controlword, directly or after decoding.

FIG. 1 is a block diagram of a conventional phase shifter unit cell 100.Two ports, P1, P2, either of which may be an input port to the phaseshifter cell 100 for an RF signal or an output port for the phaseshifter cell 100, are coupled by a low pass filter (LPF) path 102 and ahigh pass filter (HPF) path 104. In the illustrated example, the LPFpath 102 includes an LPF circuit 106, a primary LPF isolation switch108, and an optional secondary LPF isolation switch 110 (the optionalnature is symbolized by the dotted box). A common control signal, A, iscoupled to the primary LPF isolation switch 108 and the optionalsecondary LPF isolation switch 110 (if present), and may be coupled tothe LPF circuit 106 (the optional nature of such a coupling issymbolized by a dotted connection line). Similarly, the HPF path 104includes an HPF circuit 112, a primary HPF isolation switch 114, and anoptional secondary HPF isolation switch 116. A common control signal, Ā(i.e., the inverse or complement of the A control signal), is coupled tothe primary HPF isolation switch 114 and the optional secondary HPFisolation switch 116 (if present), and may be coupled to the HPF circuit112.

For small phase shifts, the LPF circuit 106 may be as simple as aninductor and the HPF circuit 112 may be as simple as a capacitor. Formedium to large phase shifts, the LPF circuit 106 and the HPF circuit112 may be more complex. For example, FIG. 2 is a schematic diagram ofan LPF circuit 200 having a conventional Pi-type configuration. Aninductor L1 provides a series connection between ports LP1, LP2, whilebracketing capacitors C1, C2 connect ports LP1 and LP2 to circuit groundthrough corresponding FET switches M1, M2. In this example, the switchesM1, M2 are controlled by the control signal A from FIG. 1, and helpprovide isolation when the LPF circuit 200 is to be isolated from portsP1 and P2. The operation of a Pi-type low pass filter is well-known inthe art.

Similarly, FIG. 3 is a schematic diagram of an HPF circuit 300 having aconventional T-type configuration. Series-connected capacitors C3, C4provide a series connection between ports HP1, HP2, while an interposedinductor L provides a connection from the junction of capacitors C3 andC4 to circuit ground through a corresponding FET switch M3. In thisexample, the switch M3 is controlled by the control signal Ā from FIG.1, and helps provide isolation when the HPF circuit 300 is to beisolated from ports P1 and P2. The operation of a T-type high passfilter is well-known in the art.

Referring to FIG. 1, all of the isolation switches are typicallyimplemented as field effect transistors (FETs) having a “CLOSED” or “ON”state (i.e., low impedance, signal conducting) and an “OPEN” or “OFF”state (i.e., high impedance, signal blocking) between the drain andsource terminals, determined by a control signal to the gate of the FET.A conventional driver circuit (not shown) concurrently generatessuitable voltages corresponding to the complementary control signals Aand Ā for the isolation switch FETs and for the FETs within thoseembodiments of the LPF circuit 106 and the HPF circuit 112 that includeswitches (as in FIG. 2 and FIG. 3); such driver circuits are also knownas level shifters. In some configurations, a bypass path (not shown) maybe included that simply connects ports P1 and P2 through a FET switchwhile setting the LPF path 102 and the HPF path 104 to an isolationstate.

In operation, the control signals A and Ā emanate from the same drivercircuit and are complementary, meaning that they flip binary states inunison: when A=1, then Ā=0, and when A=0, then Ā=1. Accordingly, onlyone of the LPF path 102 and the HPF path 104 are coupled between portsP1 and P2 at any one time.

In particular, when the LPF path 102 is to be coupled between ports P1and P2, then A=1 and Ā=0. Thus, the primary LPF isolation switch 108 andthe optional secondary LPF isolation switch 110 (if present) are ON, asare any FET switches within the LPF circuit 106. Concurrently, theprimary HPF isolation switch 114 and the optional secondary HPFisolation switch 116 (if present) are OFF, as are any FET switcheswithin the HPF circuit 112.

Conversely, when the HPF path 104 is to be coupled between ports P1 andP2, then A=0 and Ā=1. Thus, the primary HPF isolation switch 114 and theoptional secondary HPF isolation switch 116 (if present) are ON, as areany FET switches within the HPF circuit 112. Concurrently, the primaryLPF isolation switch 108 and the optional secondary LPF isolation switch110 (if present) are OFF, as are any FET switches within the LPF circuit106.

For small phase shifts (e.g., less than about 12°), using a simpleinductor for the LPF circuit 106 and a simple capacitor for the HPFcircuit 112 provides good return loss (theoretically less than about 20dB) and consumes little integrated circuit (IC) die area with fewcomponents and switches (in general, the optional secondary isolationswitches 110, 116 are not needed).

For medium phase shifts (e.g., about 12° to about 90°), the more complexLPF circuit 200 and HPF circuit 300 of FIG. 2 and FIG. 3, respectively,may be necessary to maintain good return loss and achieve targeted phaseshift states with realistic component values suitable for ICimplementation. However, the optional secondary isolation switches 110,116 generally would not be needed, thus saving IC die area.

For larger phase shifts (e.g., above about 90°), the more complex LPFcircuit 200 and HPF circuit 300 of FIG. 2 and FIG. 3, respectively, aregenerally necessary to maintain good return loss and achieve targetedphase shift states with realistic component values suitable for ICimplementation. In addition, the optional secondary isolation switches110, 116 generally would be needed to provide adequate isolation.

For a system requiring a high resolution phase shifter, such as for RFdomain cancellation or calibration, a high-isolation mode may be desiredfor a phase shifter unit cell 100 or a connected set of such cells. Forexample, in some applications, it may be a design criterion for a phaseshifter to have a disabled, or all OFF state, so that the output porthas 50 dB or more of isolation from the input port. A conventionalsolution would be to insert a FET switch in series with the input of thephase shifter chain—for example, at node X in FIG. 1—but doing so wouldintroduce added insertion loss. More specifically, in RF circuits, thepresence of a FET switch may have significant effects on the rest of thecircuit, particularly with respect to termination impedance andisolation levels. Such effects arise because an “ON” (low impedance) FEThas a non-zero resistance, R_(ON), and an “OFF” (high impedance) FETbehaves as a capacitor with capacitance C_(OFF). Thus, adding a seriesFET switch to a phase shifter circuit will increase insertion loss dueto the ON resistance, R_(ON), of the FET while remaining capacitivelycoupled to external circuitry—and thus not fully isolated from the RFsignal path—due to the capacitance C_(OFF).

Accordingly, there is a need for a phase shifter unit cell or aconnected set of such cells that can be well isolated from externalcircuitry and which does not introduce insertion loss into the RF signalpath. The present invention addresses this need and provides additionaladvantages.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a phase shifter unit cellor a connected set of such cells that can be well isolated from externalcircuitry and which do not introduce insertion loss into the RF signalpath, exhibit good return loss, and further provides additionaladvantages when combined with bracketing attenuator circuits.

More particularly, embodiments of the present invention integrate ahigh-isolation function within a phase shifter circuit that does notintroduce added insertion loss and reduces die area compared to aconventional series switch solution. These advantages are accomplishedby breaking the complimentary nature of the control signals to a phaseshifter cell to provide greater control of switch states internal to thephase shifter cell and thus enable a distinct high-isolation state, andby including a switchable shunt termination resistor for use in thehigh-isolation state.

More specifically, embodiments of the present invention include multipleindependent switch control signals corresponding to two or moreswitchable elements in a phase shifter circuit, rather than usingcomplimentary control signals to toggle between only a high pass filterstate and a low pass filter state.

Some embodiments are serially coupled to attenuator circuits to enablesynergistic interaction that reduces overall die size and/or increasesisolation. One such embodiment positions a phase shifter cell inaccordance with the present invention between bracketing programmableattenuators.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a conventional phase shifter unit cell.

FIG. 2 is a schematic diagram of an LPF circuit having a conventionalPi-type configuration.

FIG. 3 is a schematic diagram of an HPF circuit having a conventionalT-type configuration.

FIG. 4 is a block diagram of a first embodiment of the present inventionshowing a phase shifter unit cell having independent path selectioncontrol signals A, B, and a distinct isolation circuit control signal S.

FIG. 5A shows a FET device in an ON or conducting mode (i.e., Vgs>Vthfor an enhancement mode N-channel device), along with a closed switchrepresentation and the equivalent circuit (i.e., the R_(ON) resistanceof the FET device).

FIG. 5B shows a FET device in an OFF or blocking mode (i.e., Vgs=0 foran enhancement mode N-channel device), along with an open switchrepresentation and the equivalent circuit (i.e., the C_(OFF) capacitanceof the FET device).

FIG. 6 is an equivalent circuit diagram of a second phase shifter unitcell embodiment in accordance with the present invention, with switchesconfigured for the isolation mode.

FIG. 7 is an equivalent circuit diagram of a third phase shifter unitcell embodiment in accordance with the present invention, with switchesconfigured for the isolation mode.

FIG. 8 is an equivalent circuit diagram of a fourth phase shifter unitcell embodiment in accordance with the present invention, with switchesconfigured for the isolation mode.

FIG. 9 is an equivalent circuit diagram of a fifth phase shifter unitcell embodiment in accordance with the present invention, with switchesconfigured for the isolation mode.

FIG. 10 is an equivalent circuit diagram of a sixth phase shifter unitcell embodiment in accordance with the present invention, with switchesconfigured for the isolation mode.

FIG. 11 is a block diagram of a variable phase shifter seriallyconnected to a variable attenuator.

FIG. 12 is a block diagram of a variable phase shifter in accordancewith the present invention configured between bracketing seriallyconnected variable attenuators.

FIG. 13 is a block diagram of a variant of the embodiment shown in FIG.12 in which a variable phase shifter in accordance with the presentinvention is configured between bracketing serially connected variableattenuators.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a phase shifter unit cellor a connected set of such cells that can be well isolated from externalcircuitry, exhibit good return loss, and which do not introduceinsertion loss into the RF signal path, and further provides additionaladvantages when combined with bracketing attenuator circuits.

More particularly, embodiments of the present invention integrate ahigh-isolation function within a phase shifter circuit that does notintroduce added insertion loss and reduces die area compared to aconventional series switch solution. These advantages are accomplishedby breaking the complimentary nature of the control signals to a phaseshifter cell to provide greater control of switch states internal to thephase shifter cell and thus enable a distinct high-isolation state, andby including a switchable shunt termination resistor for use in thehigh-isolation state.

More specifically, embodiments of the present invention include multipleindependent switch control signals corresponding to two or moreswitchable elements in a phase shifter circuit, rather than usingcomplimentary control signals to toggle between only a high pass filterstate and a low pass filter state.

First Example Embodiment

FIG. 4 is a block diagram of a first embodiment of the present inventionshowing a phase shifter unit cell 400 having independent path selectioncontrol signals A, B, and a distinct isolation circuit control signal S.The components and configuration of the phase shifter cell 400 issimilar to the phase shifter unit cell 100 of FIG. 1, but furtherincludes a termination circuit 401 coupled to a node X on the input port(P1 in this example) of the phase shifter unit cell 400.

In the illustrated embodiment, the termination circuit 401 comprises ashunt resistor R serially coupled to a shunt switch 402, shown as a FET(such as a MOSFET). The order of the shunt resistor R and the shuntswitch 402 may be reversed. In some embodiments, the shunt resistor Rmay be a variable resistor to allow tuning the termination circuit 401to a particular characteristic impedance (typically 50 ohms for an RFcircuit). The shunt switch 402 may be implemented as a “stack” ofserially connected components (e.g., FETs) to withstand greatervoltages.

For the illustrated example, separate driver circuits (not shown) areconfigured to output independent path selection control signals A, B,and a distinct isolation circuit control signal S. As illustrated, pathselection control signal A is coupled to all of the switchable elementsof the LPF path 102 (i.e., the primary LPF isolation switch 108, theoptional secondary LPF isolation switch 110 if present, and anyswitchable elements in the LPF circuit 106). Similarly, path selectioncontrol signal B is coupled to all of the switchable elements of the HPFpath 104 (i.e., the primary HPF isolation switch 114, the optionalsecondary HPF isolation switch 116 if present, and any switch elementsin the HPF circuit 112). Isolation circuit control signal S is coupledto the shunt switch 402 of the termination circuit 401.

While the switchable elements in the example shown in FIG. 4 areillustrated as FETs, the invention is not limited to phase shifterscircuits implemented with FETs, and applies to other types of switchableelements, such as bipolar transistors, micro-electromechanical systems(MEMS) switches, etc.

In operation, three modes are generally used in accordance with thetruth table 404 shown adjacent to the phase shifter unit cell 400:

-   -   LPF mode: Path selection control signal A is set to binary 1 to        activate the LPF path 102, while path selection control signal B        is set to binary 0 to deactivate the HPF path 104 and isolation        circuit control signal S is set to binary 0 to de-couple the        shunt resistor R from node X.    -   HPF mode: Path selection control signal B is set to binary 1 to        activate the HPF path 104, while path selection control signal A        is set to binary 0 to deactivate the LPF path 102 and isolation        circuit control signal S is set to binary 0 to de-couple the        shunt resistor R from node X.    -   Isolation mode: Isolation circuit control signal S is set to        binary 1 to couple the shunt resistor R to node X, while path        selection control signal A is set to binary 0 to deactivate the        LPF path 102 and path selection control signal B is set to        binary 0 to deactivate the HPF path 104. Deactivating both the        LPF path 102 and the HPF path 104 while shunting node X to        ground provides excellent isolation of the phase shifter unit        cell 400 (and any phase shifter unit cells further along the RF        signal path from the input port) without interposing additional        impedance in the RF signal path that would increase insertion        loss. Setting isolation circuit control signal S to binary 0        enables a general “phase shifting mode” that allows the LPF mode        or the HPF mode to be selected.

As shown in the truth table 404, the state of isolation circuit controlsignal S may be generated by applying the logical NOR function to theselection control signals A and B (noting that the fourth combination,with A and B both a logical 1, would not generally be used, and thus thestate of isolation circuit control signal S is shown as “X”, meaning“don't care”). However, in other embodiments, isolation circuit controlsignal S may be separately controlled independent of path selectioncontrol signals A and B to achieve the same results.

Further Examples

Other configurations of the invention may be useful for a variety ofapplications. To simplify the figures showing such configurations,equivalent symbols are used in some of the following illustrations. Morespecifically, FIG. 5A shows a FET device 502 a in an ON or conductingmode (i.e., Vgs>Vth for an enhancement mode N-channel device), alongwith a closed switch representation 504 a and the equivalent circuit 506a (i.e., the R_(ON) resistance of the FET device 502 a). Similarly, FIG.5B shows a FET device 502 b in an OFF or blocking mode (i.e., Vgs=0 foran enhancement mode N-channel device), along with an open switchrepresentation 504 b and the equivalent circuit 506 b (i.e., the C_(OFF)capacitance of the FET device 502 b). However, there are multipletechnologies and means for implementing the illustrated switches. Forexample, focusing on FET technology alone, embodiments of the inventionmay be implemented with NMOS, PMOS, and/or CMOS transistor switches, andenhancement mode or depletion mode transistor devices. Further, suchswitches are not limited to devices having only the two states of ON andOFF. In addition, as noted above, the inventive concepts apply to othertypes of switchable elements, such as bipolar transistors,micro-electromechanical systems (MEMS) switches, etc. Accordingly, howsuch switches are shown implemented in the illustrated embodimentsshould not detract from the spirit of the invention.

FIG. 6 is an equivalent circuit diagram of a second phase shifter unitcell 600 embodiment in accordance with the present invention, withswitches configured for the isolation mode. Similar in most respects tothe embodiment shown in FIG. 4, the second phase shifter unit cell 600shows a Pi-type LPF circuit 106 having internal switches controlled bypath selection control signal A, and a T-type HPF circuit 112 having aninternal switch controlled by path selection control signal B. For theisolation mode, path selection control signals A and B are both set tobinary 0, thus opening all switches to which those control signals areconnected. For the LPF mode, path selection control signal A is set tobinary 1 and path selection control signal B is set to binary 0, and forthe HPF mode, those states are reversed.

A first resistor/shunt switch element 602 comprising a shunt switch 604serially connected to a resistor R is coupled as a switchabletermination circuit between the input port (P1 in this example) andcircuit ground. The shunt switch 604 is controlled by isolation circuitcontrol signal S and shunts the input port (P1 in this example) of thesecond phase shifter unit cell 600 to circuit ground through theresistor R when S=1. When S=0, the resistor R is disconnected from theport and the shunt switch 604 if effectively a capacitor. Accordingly,the resistor/shunt switch element 602 imposes no significant load on thesignal path from P1 to P2 and thus does not add to the insertion loss ofthe phase shifter unit cell 600. For even further isolation at the otherport, P2, an optional second resistor/shunt switch element 606 alsocontrolled by isolation circuit control signal S may be coupled as aswitchable termination circuit between the output port (P2 in thisexample) and circuit ground. In the illustrated example, the firstresistor/shunt switch element 602 and the second resistor/shunt switchelement 606 may be considered to be in an “outside” configuration,utilizing no other components of the second phase shifter unit cell 600.Note also that that the resistor/shunt switch elements 602, 606 need notbe electrically identical. For example, the sizes of their internalshunt switch FETs may differ to provide a more uniform impedance levelfor the corresponding ports port P1, P2, taking into account suchfactors as asymmetric amounts of parasitic elements (e.g., capacitancesand inductances) associated with the circuitry and layout affecting eachport. Accordingly, it may be useful to size the internal shunt switchFETs so as to offset such asymmetry and provide a more uniform signalpath impedance level.

As in the first embodiment of a phase shifter unit cell 400 shown inFIG. 4, isolation circuit control signal S for the embodiment of FIG. 6may be generated according to truth table 404 (see FIG. 4) orindependently provided as a control signal from a source external to theillustrated circuit.

FIG. 7 is an equivalent circuit diagram of a third phase shifter unitcell 700 embodiment in accordance with the present invention, withswitches configured for the isolation mode. As in FIG. 6, the thirdphase shifter unit cell 700 shows a Pi-type LPF circuit 106 havinginternal switches controlled by path selection control signal A, and aT-type HPF circuit 112 having an internal switch controlled by pathselection control signal B. For the isolation mode, path selectioncontrol signals A and B are both set to binary 0, thus opening allswitches to which those control signals are connected.

In contrast to FIG. 6, the HPF path 104 has yet another independent pathselection control signal, C, which is set to binary 1 in the illustratedisolation mode example, thus closing isolation switches 114 and 116. Oneor more resistor/shunt switch elements 702, 704 controlled by isolationcircuit control signal S are connected to the HPF path 104 between anassociated isolation switch 114, 116 and the HPF circuit 112. Asillustrated, S=1, and thus if resistor/shunt switch element 702 ispresent, any signal present at port P1 would be shunted to groundthrough isolation switch 114 and resistor/shunt switch element 702.Similarly, if resistor/shunt switch element 704 is present, any signalpresent at port P2 would be shunted to ground through isolation switch116 and resistor/shunt switch element 704. Both resistor/shunt switchelements 702, 704 may be implemented for increased isolation at bothports P1 and P2. In the illustrated example, the resistor/shunt switchelements 702, 704 may be considered to be in an “inside” configuration,since they utilize the isolation switches 114 and 116 to complete atermination path to ground when in the isolation mode.

A first variant configuration would independently control isolationswitches 108 and 110 with a path selection control signal D and couplecorresponding resistor/shunt switch elements to the LPF path 102 betweenthe isolation switches 108, 110 and the LPF circuit 106 (i.e.,essentially mirroring the configuration shown in FIG. 7, but using theLPF path 102 rather than the HPF path 104 for isolation). A secondvariant configuration would use both the illustrated circuit of FIG. 7and the first variant configuration just described, placingresistor/shunt switch elements (not shown) on both the LPF path 102 andthe HPF path 104 and closing isolation switches 108, 110, 114, and 116in the isolation mode with path selection control signals C and D. Ineither case, each resistor/shunt switch element 702, 704 is controlledby isolation circuit control signal S.

In any variant of the third phase shifter unit cell 700 of FIG. 7, theLPF mode and the HPF mode are enabled by setting isolation circuitcontrol signal S=0 for each resistor/shunt switch element, and havingpath selection control signal C track path selection control signal B(i.e., C=B) and path selection control signal D track path selectioncontrol signal A (i.e., D=A). However, as described above, in theisolation mode, path selection control signals C and D are controlledindependently of path selection control signals A and B.

One advantage of the configuration shown in FIG. 7 and its variants isthat the use of one or more of the existing isolation switches 108, 110,114, and 116 in series combination with a corresponding resistor/shuntswitch element allows the shunt resistor R to be smaller (since theR_(ON) resistance of the isolation switches withstands some signalpower), or provides higher power handling capability if a “normal” sizedresistor is used for R (since R_(ON) is added to R).

Aspects of both the “outside” configuration of FIG. 6 and the “inside”configuration of FIG. 7 can be combined. For example, an “outside”configuration can be used for coupling a resistor/shunt switch elementto port P1, while an “inside” configuration can be used for coupling aresistor/shunt switch element to port P2, with suitable independent pathselection control signals and an isolation circuit control signal S.Thus, referring to FIG. 7, resistor/shunt switch element 702 may beshifted to the port P1 side of isolation switch 114, in which case pathselection control signal B would be used to control isolation switch114, as in FIG. 6.

FIG. 8 is an equivalent circuit diagram of a fourth phase shifter unitcell 800 embodiment in accordance with the present invention, withswitches configured for the isolation mode. As in FIG. 6, the fourthphase shifter unit cell 800 shows a Pi-type LPF circuit 106 havinginternal switches controlled by path selection control signal A, and aT-type HPF circuit 112 having an internal switch controlled by pathselection control signal B. As illustrated, path selection controlsignal B is set to binary 0, thus opening all switches to which thatcontrol signal is connected, while independent path selection controlsignal A is set to binary 1, thus closing all switches to which thatcontrol signal is connected. One or more resistor/shunt switch elements802 are coupled as shown between a corresponding internal switch 806,808 of the Pi-type LPF circuit 106 and circuit ground. Eachresistor/shunt switch element 802 is controlled by isolation circuitcontrol signal S.

One advantage of the configuration shown in FIG. 8 is that the use ofone or more of the existing isolation switches 108, 110 in series withan internal switch 806, 808 of the Pi-type LPF circuit 106 allows theshunt resistor R of the corresponding series-connected resistor/shuntswitch element 802 to be even smaller than the circuit configuration ofFIG. 7 (since the R_(ON) of the isolation switches and the R_(ON) of theinternal switches withstands more signal power than the single seriesisolation switch configuration of FIG. 7), or provides higher powerhandling capability if a “normal” sized resistor is used for R (sinceapproximately 2×R_(ON) is added to R).

FIG. 9 is an equivalent circuit diagram of a fifth phase shifter unitcell 900 embodiment in accordance with the present invention, withswitches configured for the isolation mode. As in FIG. 6, the fifthphase shifter unit cell 900 shows a Pi-type LPF circuit 106 havinginternal switches 902, 904, a T-type HPF circuit 112 having an internalswitch 906, an LPF path 102 having isolation switches 108, 110, and anHPF path 104 having isolation switches 114, 116. Also included are tworesistor/shunt switch elements 908, 910 controlled by the isolationcircuit control signal S. Resistor/shunt switch element 908 isconfigured similar to resistor/shunt switch element 802 of FIG. 8, whileresistor/shunt switch element 910 is configured similar toresistor/shunt switch element 704 of FIG. 7.

In the example illustrated in FIG. 9, each switch is independentlycontrolled (although in some cases, the state of one switch controlsignal may exactly track the state of another switch control signal,thus allowing some simplification of the control logic). For the circuitconfiguration illustrated in FIG. 9, the switch control states forvarious operational modes are as shown in TABLE 1.

TABLE 1 Control State by Mode Control Signal LPF Mode HPF Mode IsolationMode A 1 0 1 B 0 1 0 C 1 0 0 D 0 1 1 E 1 0 1 F 1 0 0 G 0 1 0 S 0 0 1

Note that control signals A and E track each other, as do controlsignals B and G and control signals C and F. Accordingly, TABLE 1 can besimplified as shown in TABLE 2.

TABLE 2 Control State by Mode Control Signal LPF Mode HPF Mode IsolationMode A, E 1 0 1 B, G 0 1 0 C, F 1 0 0 D 0 1 1 S 0 0 1

FIG. 10 is an equivalent circuit diagram of a sixth phase shifter unitcell 1000 embodiment in accordance with the present invention, withswitches configured for the isolation mode. As in FIG. 6, the fifthphase shifter unit cell 900 shows a Pi-type LPF circuit 106 havinginternal switches 1002, 1004, a T-type HPF circuit 112 having aninternal switch 1006, an LPF path 102 having isolation switches 108,110, and an HPF path 104 having isolation switches 114, 116. Alsoincluded are two resistor/shunt switch elements 1008, 1010 controlled bythe isolation circuit control signal S. Resistor/shunt switch element1008 is configured similar to resistor/shunt switch element 604 of FIG.6. However, resistor/shunt switch element 1010 is serially coupledbetween port P1 and circuit ground through the internal switch 1006 andinductor L of the T-type HPF circuit 112, thus providing a “series”shunt path that does not impose appreciable insertion loss when in theHPF or LPF modes at sufficiently low RF frequencies, yet provides atermination to ground when in the isolation mode (at higher RFfrequencies, the frequency dependent impedance of the inductor L inseries with the resistor R of the resistor/shunt switch element 1010when the phase shifter unit cell 1000 is in the isolation mode may needto be taken into account during design).

For the circuit configuration illustrated in FIG. 10, the switch controlstates for various operational modes are as shown in TABLE 3.

TABLE 3 Control State by Mode Control Signal LPF Mode HPF Mode IsolationMode A 1 0 0 B 0 1 0 C 0 1 1 S 0 0 1

As the phase shifter unit cell examples shown in FIGS. 4 and 6-10illustrate, the concepts of the present invention can be implemented ina wide variety of embodiments, which generally include a distinctisolation circuit control signal for controlling at least onetermination circuit path operationally coupled between a correspondingport and circuit ground, and at least two independently selectable pathselection control signals for controlling at least two correspondingswitches within separate phase shift signal paths. Such phase shifterunit cells thus can be well isolated from external circuitry withoutintroducing any significant insertion loss into the RF signal path butstill exhibiting good return loss.

Further, the invention extends to low-insertion loss isolation of phaseshifter circuits having more than the two signal paths (LPF and HPF),such as phase shifters utilizing multi-state phase shifters of the typetaught in U.S. patent application Ser. No. 15/017,433, entitled “LowLoss Multi-State Phase Shifter”, filed Feb. 5, 2016, and assigned to theassignee of the present invention, the contents of which are herebyincorporated by reference. In such a case, at least one terminationcircuit path would be operationally coupled between a corresponding portand circuit ground and controlled by a distinct isolation circuitcontrol signal. In an isolation mode, each signal path is either takenout of circuit by a corresponding path selection control signal, orserially connected (at least in part) with a termination circuit path.In operational phase shift modes, the termination circuit path is takenout of circuit so as to present little or no load on any RF signal path.

If several phase shifter unit cells are serially connected so as toprovide a wider range of selectable phase shifts, each phase shifterunit cell may include a termination circuit path controlled by theisolation circuit control signal. Alternatively, such a terminationcircuit path may be included only in one or both of the end-most phaseshifter unit cells in the series, thereby saving IC die area.

For many applications, it may be useful to serially-connect multiplephase shifter unit cells of the same or different phase shift values.Such phase shifter circuits may be digitally controlled and thus providea discrete set of phase states that are selected by a binary controlword, directly or after decoding. For example, such phase shiftercircuits may be binary-coded, thermometer coded, or a hybrid combinationof the two types.

Phase Shifter and Attenuator Combinations

Another aspect of the invention includes serially connecting one or moredigitally selectable phase shifter unit cells having at least onetermination circuit path as described above to one or more digitallyselectable RF signal attenuator circuits that provides a discrete set ofattenuation states that are selected by a binary control word, directlyor after decoding. Such attenuation/phase shifter circuit configurationsenable synergistic interaction that reduces overall die size and/orincreases isolation.

For example, FIG. 11 is a block diagram 1100 of a variable phase shifter1102 serially connected to a variable attenuator 1104. In theillustrated example, the variable attenuator 1104 includes an RF_In portand the variable phase shifter 1102 includes an RF_Out port. Thevariable phase shifter 1102 may comprise one or more digitallyselectable phase shifter unit cells of the types described above, andthe variable attenuator 1104 may comprise one or more digitallyselectable attenuator unit cells. Examples of variable attenuators 1104are disclosed in U.S. patent application Ser. No. 14/084,439, entitled“Segmented Attenuator with Glitch Reduction”, filed Nov. 19, 2013, andin U.S. patent application Ser. No. 14/878,750, entitled “ImprovedMulti-State Attenuator”, filed Oct. 8, 2015, both of which are assignedto the assignee of the present invention, the contents of which arehereby incorporated by reference. Another input to the variable phaseshifter 1102 is a set of independent switch control signals 1110 thatprovide normal operational control of the component digitally selectablephase shifter unit cells, as described above.

FIG. 11 shows the inclusion of a first termination circuit 1106comprising a resistor/shunt switch element, and an optional secondtermination circuit 1108 comprising a resistor/shunt switch element,both controlled by an isolation circuit control signal S; asillustrated, the first termination circuit 1106 and the optional secondtermination circuit 1108 are in the isolation mode. While the firsttermination circuit 1106 could be coupled to the RF signal path beforethe variable attenuator 1104 (e.g., at node X), it is moreadvantageously positioned between the variable phase shifter 1102 andthe variable attenuator 1104. By placing the first termination circuit1106 on the RF signal path with the variable attenuator 1104 interposedwith respect to the RF_In port, the variable attenuator 1104 can reducethe power handling requirement of the first termination circuit 1106 inthe isolation mode, since some signal power is absorbed within thevariable attenuator 1104. Such a configuration reduces the IC die arearequired for the resistor and switch stack of the termination circuit1106. Further, in the isolation mode, the variable attenuator 1104 canbe programmatically set to its maximum attenuation value to furtherimprove the isolation of the RF_Out port from the RF_In port. Theoptional second termination circuit 1108 is coupled to the other port(here, RF_Out) of the variable phase shifter 1102 to still furtherimprove the isolation of the RF_Out port from the RF_In port.

The circuit illustrated in FIG. 11 is asymmetric, in that the variableattenuator 1104 only “protects” the first termination circuit 1106 fromapplied signal power in the isolation mode if interposed between thefirst termination circuit 1106 and the RF_In port. FIG. 12 is a blockdiagram 1200 of a variable phase shifter 1102 in accordance with thepresent invention configured between bracketing serially connectedvariable attenuators 1104 a, 1104 b. First and second terminationcircuits 1106, 1108 are connected to the signal path from port P1 toport P2 and are both controlled by an isolation circuit control signalS; as illustrated, the first and second termination circuits 1106, 1108are in the isolation mode. The variable attenuators 1104 a, 1104 b mayhave the same range of attenuation as the single variable attenuator1104 of FIG. 11, or each may have a lesser range of attenuation as thesingle variable attenuator 1104 of FIG. 11 (e.g., one-half). Anotherinput to the variable phase shifter 1102 is a set of independent switchcontrol signals 1110 that provide normal operational control of thecomponent digitally selectable phase shifter unit cells, as describedabove.

An advantage of the configuration shown in FIG. 12 is that eachtermination circuit 1106, 1108 is “protected” from the input power ofany signal applied to port P1 or to port P2 by an intervening variableattenuator 1104 a, 1104 b.

In some applications, the input return loss may be acceptable withoutrequiring independently controlled termination circuits 1106, 1108 forthe embodiments shown in FIG. 11 and FIG. 12, so long as there areindependent switch control signals 1110 such that the various switcheswithin the variable phase shifter 1102 can be set to a state thatprovides for isolation. For example, FIG. 13 is a block diagram 1300 ofa variant of the embodiment shown in FIG. 12 in which a variable phaseshifter 1302 in accordance with the present invention is configuredbetween bracketing serially connected variable attenuators 1104 a, 1104b. As in the embodiments of FIG. 11 and FIG. 12, the variable phaseshifter 1302 may comprise one or more digitally selectable phase shifterunit cells of the types described above, and the variable attenuators1104 a, 1104 b may comprise one or more digitally selectable attenuatorunit cells. The variable phase shifter 1302 and the variable attenuators1104 a, 1104 b may be binary-coded, thermometer coded, or a hybridcombination of the two types.

An input to the variable phase shifter 1302 is a set of independentswitch control signals 1110 that provide normal operational control ofthe component digitally selectable phase shifter unit cells, asdescribed above; in the illustrated example, such switch control signalsinclude at least independent “A” and “B” signals.

The circuit shown in FIG. 13 is similar to a combination of the circuitshown in FIG. 4 (for the variable phase shifter 1302) and the circuitshown in FIG. 12 (for the combination of variable attenuators 1104 a,1104 b and a variable phase shifter). However, when an isolation mode isrequired, the “A” and “B” signals are both set to a binary 0, thusopening respective switches in both the LPF signal path and the HPFsignal path between Port1 and Port2 and providing a reasonable degree ofisolation (in contrast, in a conventional phase shifter, the switches inthe LPF signal path and the HPF signal path cannot all be openedconcurrently). For even more isolation, the variable attenuators 1104 a,1104 b can be programmatically set to their respective maximumattenuation values to further improve the isolation of the P1 port fromthe P2 port. Because the switch control signals 1110 are independent andthus enable concurrently isolating the signal paths within the variablephase shifter 1302, termination circuits 1106, 1108 (as in FIG. 11 andFIG. 12) are not required if the measured input return loss isacceptable for particular applications.

Notably, the external return loss would be about twice the maximumattenuation value of either of the attenuators 1104 a, 1104 b (dependingon the direction of signal propagation) if loaded with a high impedanceby setting the variable phase shifter 1302 to the isolation (opencircuit) mode. That is, an open-ended attenuator can provide two timesits attenuation value (“X”) as the return loss. Thus, an input signal isattenuated by X amount, reflected by the open circuit, and thenattenuated by another X amount; X+X=2X, which is the definition of“return loss.” This is effectively like a resistive load, from the pointof view of an external signal. Further, omitting the independentlycontrolled termination circuits may reduce parasitic loading of thesignal path in normal operation and improve performance. As similarresult may pertain to the circuit configurations shown in FIG. 11 andFIG. 12.

Methods

Another aspect of the invention includes a method for shifting the phaseof an applied signal, including: providing at least two phase shiftsignal paths, each coupled to first and second ports, for providing aphase shift to a signal applied to at least one of the first and secondports and responsive to a corresponding independent path selectioncontrol signal for selectively independently enabling communication ofthe applied signal from the first port to the second port through thecorresponding phase shift signal path when not in an isolation mode, andfor disabling communication of the applied signal from the first port tothe second port through the corresponding phase shift signal path whenin the isolation mode; and providing at least one selectable terminationcircuit, each operatively coupled to a corresponding one of the first orsecond ports, and responsive to a distinct isolation circuit controlsignal for isolating the first port from the second port in theisolation mode.

Yet another aspect of the invention includes a method for shifting thephase of an applied signal, including: providing at least two phaseshift signal paths, each for providing a phase shift to a signal appliedto at least one of the first and second ports; coupling at least twoindependent path selection control signals to a corresponding phaseshift signal path; coupling a selectable termination circuit to acorresponding one of the first or second ports; selectively enablingeach selectable termination circuit to isolate the first port from thesecond port in an isolation mode; selectively disabling each selectabletermination circuit when not in an isolation mode; selectivelyindependently enabling communication of the applied signal from thefirst port to the second port through the corresponding phase shiftsignal path when not in the isolation mode; and disabling communicationof the applied signal from the first port to the second port through thecorresponding phase shift signal path when in the isolation mode.

Still another aspect of the invention includes a method for shifting thephase of an applied radio frequency signal, including: providing atleast two phase shift signal paths, each including at least one phaseshifter element serially coupled through at least one isolation switchto first and second ports, for providing a selectable degree of phaseshift to a radio frequency signal applied to at least one of the firstand second ports; providing at least two independent path selectioncontrol signals, each coupled to a corresponding phase shift signalpath, for selectively coupling the corresponding phase shift signal pathto the first and second ports; providing at least one selectabletermination circuit, each coupled between circuit ground and acorresponding one of the first or second ports; selectively switchingthe at least one termination circuit with a distinct isolation circuitcontrol signal to an isolation mode to operationally couple thecorresponding one of the first or second ports to circuit ground, or toa phase shifting mode to de-couple the corresponding one of the first orsecond ports from circuit ground; and in the isolation mode, setting theat least two independent path selection control signals to states thateffect isolation of the corresponding phase shift signal path from thefirst and second ports.

Other aspects of such methods include deriving the distinct isolationcircuit control signal from a logical combination of the independentpath selection control signals, and providing embodiments of andconnections for the phase shift signal paths and termination circuits inaccordance with the teachings above.

Fabrication Technologies and Options

The term “MOSFET” technically refers to metal-oxide-semiconductors;another synonym for MOSFET is “MISFET”, formetal-insulator-semiconductor FET. However, “MOSFET” has become a commonlabel for most types of insulated-gate FETs (“IGFETs”). Despite that, itis well known that the term “metal” in the names MOSFET and MISFET isnow often a misnomer because the previously metal gate material is nowoften a layer of polysilicon (polycrystalline silicon). Similarly, the“oxide” in the name MOSFET can be a misnomer, as different dielectricmaterials are used with the aim of obtaining strong channels withsmaller applied voltages. Accordingly, the term “MOSFET” as used hereinis not to be read as literally limited to metal-oxide-semiconductors,but instead includes IGFETs in general.

As should be readily apparent to one of ordinary skill in the art,various embodiments of the invention can be implemented to meet a widevariety of specifications. Unless otherwise noted above, selection ofsuitable component values is a matter of design choice and variousembodiments of the invention may be implemented in any suitable ICtechnology (including but not limited to MOSFET and IGFET structures),or in hybrid or discrete circuit forms. Integrated circuit embodimentsmay be fabricated using any suitable substrates and processes, includingbut not limited to standard bulk silicon, silicon-on-insulator (SOI),silicon-on-sapphire (SOS), GaN HEMT, GaAs pHEMT, and MESFETtechnologies. However, the inventive concepts described above areparticularly useful with an SOI-based fabrication process (includingSOS), and with fabrication processes having similar characteristics.Fabrication in CMOS on SOI or SOS enables low power consumption, theability to withstand high power signals during operation due to FETstacking, good linearity, and high frequency operation (in excess ofabout 10 GHz, and particularly above about 20 GHz). Monolithic ICimplementation is particularly useful since parasitic capacitancesgenerally can be kept low (or at a minimum, kept uniform across allunits, permitting them to be compensated) by careful design.

Voltage levels may be adjusted or voltage and/or logic signal polaritiesreversed depending on a particular specification and/or implementingtechnology (e.g., NMOS, PMOS, or CMOS, and enhancement mode or depletionmode transistor devices). Component voltage, current, and power handlingcapabilities may be adapted as needed, for example, by adjusting devicesizes, serially “stacking” components (particularly FETs) to withstandgreater voltages, and/or using multiple components in parallel to handlegreater currents. Additional circuit components may be added to enhancethe capabilities of the disclosed circuits and/or to provide additionalfunctional without significantly altering the functionality of thedisclosed circuits.

A number of embodiments of the invention have been described. It is tobe understood that various modifications may be made without departingfrom the spirit and scope of the invention. For example, some of thesteps described above may be order independent, and thus can beperformed in an order different from that described. Further, some ofthe steps described above may be optional. Various activities describedwith respect to the methods identified above can be executed inrepetitive, serial, or parallel fashion. It is to be understood that theforegoing description is intended to illustrate and not to limit thescope of the invention, which is defined by the scope of the followingclaims, and that other embodiments are within the scope of the claims.

What is claimed is:
 1. A digitally-controlled phase shifter, including:(a) at least two phase shift signal paths, each coupled to first andsecond ports, for providing a phase shift to a signal applied to atleast one of the first and second ports and responsive to acorresponding independent path selection control signal for selectivelyindependently enabling communication of the applied signal from thefirst port to the second port through the corresponding phase shiftsignal path when not in an isolation mode, and disabling communicationof the applied signal from the first port to the second port through thecorresponding phase shift signal path when in the isolation mode; and(b) at least one selectable termination circuit, each operativelycoupled to a corresponding one of the first or second ports, andresponsive to a distinct isolation circuit control signal for isolatingthe first port from the second port in the isolation mode.
 2. The phaseshifter of claim 1, wherein the at least one selectable terminationcircuit includes a first selectable termination circuit coupled betweencircuit ground and the first port, and a second selectable terminationcircuit coupled between circuit ground and the second port.
 3. The phaseshifter of claim 1, wherein at least one phase shift signal path of theat least two phase shift signal paths includes at least one isolationswitch serially coupled to one of the first and second ports, and atleast one phase shifter element serially coupled to the at least oneisolation switch.
 4. The phase shifter of claim 3, wherein at least oneselectable termination circuit of the at least one selectabletermination circuit is coupled between circuit ground and acorresponding one of the first or second ports through at least onecorresponding isolation switch.
 5. The phase shifter of claim 3, whereinat least one selectable termination circuit of the at least oneselectable termination circuit is coupled between circuit ground and acorresponding one of the first or second ports through a correspondingphase shifter element.
 6. The phase shifter of claim 1, wherein theapplied signal is a radio frequency signal.
 7. The phase shifter ofclaim 1, wherein the distinct isolation circuit control signal isderived from a logical combination of the independent path selectioncontrol signals.
 8. A digitally-controlled phase shifter, including: (a)at least two phase shift signal paths, each coupled to first and secondports, for providing a phase shift to a signal applied to at least oneof the first and second ports; (b) at least one selectable terminationcircuit, each operatively coupled to a corresponding one of the first orsecond ports, and responsive to a distinct isolation circuit controlsignal for isolating the first port from the second port in an isolationmode; and (c) at least two independent path selection control signals,each coupled to a corresponding one of the at least two phase shiftsignal paths, each selectively independently enabling communication ofthe applied signal from the first port to the second port through thecorresponding phase shift signal path when not in the isolation mode,and each disabling communication of the applied signal from the firstport to the second port through the corresponding phase shift signalpath when in the isolation mode.
 9. The phase shifter of claim 8,wherein the at least one selectable termination circuit includes a firstselectable termination circuit coupled between circuit ground and thefirst port, and a second selectable termination circuit coupled betweencircuit ground and the second port.
 10. The phase shifter of claim 8,wherein at least one phase shift signal path of the at least two phaseshift signal paths includes at least one isolation switch seriallycoupled to one of the first and second ports, and at least one phaseshifter element serially coupled to the at least one isolation switch.11. The phase shifter of claim 10, wherein at least one selectabletermination circuit of the at least one selectable termination circuitis coupled between circuit ground and a corresponding one of the firstor second ports through at least one isolation switch.
 12. The phaseshifter of claim 10, wherein at least one selectable termination circuitof the at least one selectable termination circuit is coupled betweencircuit ground and a corresponding one of the first or second portsthrough a corresponding phase shifter element.
 13. The phase shifter ofclaim 8, wherein the applied signal is a radio frequency signal.
 14. Thephase shifter of claim 8, wherein the distinct isolation circuit controlsignal is derived from a logical combination of the independent pathselection control signals.
 15. A phase shifter including at least onephase shifter unit cell, the at least one phase shifter unit cellincluding: (a) at least two phase shift signal paths, each including atleast one phase shifter element serially coupled through at least oneisolation switch to first and second ports, for providing a selectabledegree of phase shift to a radio frequency signal applied to at leastone of the first and second ports; (b) at least two independent pathselection control signals, each coupled to a corresponding one of the atleast two phase shift signal paths, for selectively coupling thecorresponding phase shift signal path to the first and second ports; (c)at least one selectable termination circuit, each coupled betweencircuit ground and a corresponding one of the first or second ports; and(d) a distinct isolation circuit control signal, coupled to eachselectable termination circuit, for switching the at least onetermination circuit to an isolation mode to operationally couple thecorresponding one of the first or second ports to circuit ground, andfor switching the at least one termination circuit to a phase shiftingmode by de-coupling the corresponding one of the first or second portsfrom circuit ground; wherein, in the isolation mode, the at least twoindependent path selection control signals are set to states that effectisolation of the corresponding phase shift signal path from the firstand second ports.
 16. The phase shifter of claim 15, wherein the atleast one selectable termination circuit includes a first selectabletermination circuit coupled between circuit ground and the first port,and a second selectable termination circuit coupled between circuitground and the second port.
 17. The phase shifter of claim 15, whereinat least one phase shift signal path of the at least two phase shiftsignal paths includes a first isolation switch serially coupled to oneof the first and second ports, at least one phase shifter element of theat least one phase shifter element serially coupled to the firstisolation switch, and a second isolation switch serially coupled to theat least one phase shifter element of the at least one phase shifterelement and to the other of the first and second ports.
 18. The phaseshifter of claim 15, wherein at least one selectable termination circuitof the at least one selectable termination circuit is coupled betweencircuit ground and a corresponding one of the first or second portsthrough at least one isolation switch.
 19. The phase shifter of claim15, wherein at least one selectable termination circuit of the at leastone selectable termination circuit is coupled between circuit ground anda corresponding one of the first or second ports through a correspondingphase shifter element.
 20. The phase shifter of claim 15, wherein thedistinct isolation circuit control signal is derived from a logicalcombination of the at least two independent path selection controlsignals.
 21. A method for shifting the phase of an applied signal,including: (a) providing at least two phase shift signal paths, eachcoupled to first and second ports, for providing a phase shift to asignal applied to at least one of the first and second ports andresponsive to a corresponding independent path selection control signalfor selectively independently enabling communication of the appliedsignal from the first port to the second port through the correspondingphase shift signal path when not in an isolation mode, and for disablingcommunication of the applied signal from the first port to the secondport through the corresponding phase shift signal path when in theisolation mode; and (b) providing at least one selectable terminationcircuit, each operatively coupled to a corresponding one of the first orsecond ports, and responsive to a distinct isolation circuit controlsignal for isolating the first port from the second port in theisolation mode.
 22. The method of claim 21, wherein the at least oneselectable termination circuit includes a first selectable terminationcircuit coupled between circuit ground and the first port, and a secondselectable termination circuit coupled between circuit ground and thesecond port.
 23. The method of claim 21, wherein at least one phaseshift signal path of the at least two phase shift signal paths includesat least one isolation switch serially coupled to one of the first andsecond ports, and at least one phase shifter element serially coupled tothe at least one isolation switch.
 24. The method of claim 23, whereinat least one selectable termination circuit of the at least oneselectable termination circuit is coupled between circuit ground and acorresponding one of the first or second ports through at least onecorresponding isolation switch of the at least one isolation switch. 25.The method of claim 23, wherein at least one selectable terminationcircuit of the at least one selectable termination circuit is coupledbetween circuit ground and a corresponding one of the first or secondports through a corresponding one of the at least one phase shifterelement.
 26. The method of claim 21, wherein the applied signal is aradio frequency signal.
 27. The method of claim 21, further includingderiving the distinct isolation circuit control signal from a logicalcombination of the independent path selection control signals.
 28. Amethod for shifting the phase of an applied signal, including: (a)providing at least two phase shift signal paths, each for providing aphase shift to a signal applied to at least one of first and secondports; (b) coupling at least two independent path selection controlsignals to a corresponding phase shift signal path of the at least twophase shift signal paths; (c) coupling a selectable termination circuitto a corresponding one of the first or second ports; (d) selectivelyenabling each selectable termination circuit to isolate the first portfrom the second port in an isolation mode; (e) selectively disablingeach selectable termination circuit when not in an isolation mode; (f)selectively independently enabling communication of the applied signalfrom the first port to the second port through the corresponding phaseshift signal path when not in the isolation mode; and (g) disablingcommunication of the applied signal from the first port to the secondport through the corresponding phase shift signal path when in theisolation mode.
 29. The method of claim 28, wherein the at least oneselectable termination circuit includes a first selectable terminationcircuit coupled between circuit ground and the first port, and a secondselectable termination circuit coupled between circuit ground and thesecond port.
 30. The method of claim 28, wherein at least one phaseshift signal path of the at least two phase shift signal paths includesat least one isolation switch serially coupled to one of the first andsecond ports, and at least one phase shifter element serially coupled tothe at least one isolation switch.
 31. The method of claim 30, whereinat least one selectable termination circuit of the at least oneselectable termination circuit is coupled between circuit ground and acorresponding one of the first or second ports through at least oneisolation switch.
 32. The method of claim 30, wherein at least oneselectable termination circuit of the at least one selectabletermination circuit is coupled between circuit ground and acorresponding one of the first or second ports through a correspondingphase shifter element.
 33. The method of claim 28, wherein the appliedsignal is a radio frequency signal.
 34. The method of claim 28, furtherincluding deriving the distinct isolation circuit control signal from alogical combination of the independent path selection control signals.35. A method for shifting the phase of an applied radio frequencysignal, including: (a) providing at least two phase shift signal paths,each including at least one phase shifter element serially coupledthrough at least one isolation switch to first and second ports, forproviding a selectable degree of phase shift to a radio frequency signalapplied to at least one of the first and second ports; (b) providing atleast two independent path selection control signals, each coupled to acorresponding one of the at least two phase shift signal paths, forselectively coupling the corresponding phase shift signal path to thefirst and second ports; (c) providing at least one selectabletermination circuit, each coupled between circuit ground and acorresponding one of the first or second ports; (d) selectivelyswitching the at least one termination circuit with a distinct isolationcircuit control signal to an isolation mode to operationally couple thecorresponding one of the first or second ports to circuit ground, or toa phase shifting mode to de-couple the corresponding one of the first orsecond ports from circuit ground; and (e) in the isolation mode, settingthe at least two independent path selection control signals to statesthat effect isolation of the corresponding phase shift signal path fromthe first and second ports.
 36. The method of claim 35, wherein the atleast one selectable termination circuit includes a first selectabletermination circuit coupled between circuit ground and the first port,and a second selectable termination circuit coupled between circuitground and the second port.
 37. The method of claim 35, wherein at leastone phase shift signal path of the at least two phase shift signal pathsincludes a first isolation switch serially coupled to one of the firstand second ports, at least one phase shifter element of the at least onephase shifter element serially coupled to the first isolation switch,and a second isolation switch serially coupled to the at least one phaseshifter element of the at least one phase shifter element and to theother of the first and second ports.
 38. The method of claim 35, whereinat least one selectable termination circuit of the at least oneselectable termination circuit is coupled between circuit ground and acorresponding one of the first or second ports through at least oneisolation switch.
 39. The method of claim 35, wherein at least oneselectable termination circuit of the at least one selectabletermination circuit is coupled between circuit ground and acorresponding one of the first or second ports through a correspondingphase shifter element of the at least one phase shifter element.
 40. Themethod of claim 35, further including deriving the distinct isolationcircuit control signal from a logical combination of the independentpath selection control signals.